I looked up the tax records for the counties biggest and closest to the Duggars, and they don't appear to have spent anything near $10 million on real estate. Their most valuable asset appears to be the parcel of land they live on, 20 acres with an estimated value of $820,000 including the structures. Besides their home, it has a 31,000 square foot warehouse that fronts the road. The 20 acres of land excluding the structures is estimated at $40,000. Across the road is a public landfill, so it's probably not the most desirable addresses in town, but I'm not an expert on these things. They also appear to own half a dozen small houses similar to grandma's house, and a few commercial properties, the most notable being a building leased to a rental car company.
I could be wrong about all of this, I am not American and I found the system of "assessed" tax values, and property sales conducted by warrantee deeds with consideration being "$10 and other goods and valuable" very confusing.
